ARTICLE IN PRESS Journal of Cranio-Maxillofacial Surgery (2003) 31, 337–342 r 2003 European Association for Cranio-Maxillofacial Surgery. doi:10.1016/S1010-5182(03)00082-9, available online at http://www.sciencedirect.com
Er:YAG laser osteotomy directed by sensor controlled systems Stephan Rupprecht1, Katja Tangermann2, Peter Kessler1, Friedrich Wilhelm Neukam1, Joerg Wiltfang1 1
Department of Oral & Cranio-Maxillofacial Surgery (Head: Prof. Dr. Dr. Friedrich Wilhelm Neukam), Erlangen-Nuremberg University, Erlangen-Nuremberg, Germany; 2 Bayerisches Laserzentrum (Head: Prof. Dr. mult. M. Geiger), gGmbH, Konrad-Zuse-Str. 4-6, D-91052 Erlangen, Germany SUMMARY. Background: Great efforts have been taken in the past to develop laser systems suitable for bone
cutting. Laser systems emitting light in the infrared spectrum (2.9, 3.0 lm) have been found to be ideal for efficient bone ablation with very little carbonization. Aim: To evaluate a new laser bone cutting system enabling the automatic detection of different tissue qualities by an integrated sensor to avoid damage to sensitive structures such as blood vessels or nerves. Material: An Erbium:YAG laser containing an integrated closed-loop control system, was constructed and tested on dissected bone. Process emissions such as resonance changes caused by the interaction of laser light and various tissue structures can be used for a controlled system. Sensor signals from a photodiode and a piezo-electric accelerometer were received and processed to guide the laser osteotomy. Methods: Tests were performed on dissected bone specimens from rabbit femur (14) and minipig jaw (6). After laser application, the bone specimens were evaluated macroscopically and histologically. Results: The specimens were evaluated histomorphometrically for the depth of cortical bone ablation when the closed-loop control system switched off the laser. Mean courses of 97.45% (pig) and 97.83% (rabbit) showed that the systems work with precision. Conclusion: After penetrating the cortical bone layer, the laser beam was promptly interrupted due to extreme changes of the signal character received by the sensor system. The in vitro tests of this new laser closedloop control system were successful. r 2003 European Association for Cranio-Maxillofacial Surgery. Keywords: Er:YAG laser; Laser osteotomy; Animal bone; Closed-loop control system
absorption rate of the irradiated tissue must be high (Meier, 1998). Of all tested laser systems the Erbium:Yttrium AluminumGarnet (Er:YAG) laser is an ideal system for bone separation. The Er:YAG laser emits radiation at 2.94 mm. Depending on the pulse duration (150–500 ms), a bone ablation rate of up to 90 mm can be achieved. The heat-related damage to the tissue is so low that wound healing is comparable with conventional water cooled osteotomies performed with drills or oscillating saws (Lewandrowski et al., 1996; Peavy et al., 1999). However, when using a laser system it is hard to control the depth of the cut, therefore an integrated closed-loop control system was developed as a mechanism to perform safe laser osteotomy without damaging the internal and surrounding tissues. The aim of this control mechanism is to detect different tissue structures during the application of laser energy. Emissions caused by the interaction of laser-light and tissue surfaces are registered to stop laser tissue ablation as soon as the tissue characteristics change i.e. when the bone has been cut through, so as not to destroy the surrounding soft tissues. The interaction of laser and tissue causes optical and acoustic emissions such as burn- or pyrolysis lights, thermal radiation, air- and structure-borne sounds. Photodiodes, pyrometers, microphones and piezoelectric accelerometers are suitable for detecting such
BACKGROUND Osteotomies are being performed with drills, oscillating saws and chisels. However, these procedures, even if performed with great care, carry the risk of damaging the surrounding and enveloped tissues. The more complicated the anatomical location, the higher the risk of damage, especially when neural or vascular structures are situated in the direct vicinity. In cranio-maxillofacial surgery, for example, the sagittal bone split in the angle region of the mandible is a typical standard procedure. This technique carries a high risk of injury to the inferior alveolar nerve (Ja. askel ainen et al., 2000). Moreover, nerve . . injury is a well-known complication for the removal of the lower third molar (Ramadas and Sealey, 2001). Ideally, a new technique is required for bone cutting to reduce the risk of injuring the vulnerable tissues. An osteotomy could also be performed with the aid of a laser system (Biyikli and Modest, 1987; Horch, 1993; Bhatta et al., 1994; Friesen et al., 1999). The effect of such a laser is based on photoablation. Photoablation is a non-linear, photo-thermic reaction transforming irradiated tissues into a gas or plasma phase. The irradiated tissues are immediately dissolved before heat can be spread to the surrounding tissues, so reducing the thermal reaction in the direct vicinity of the laser-irradiated region. For this reason the laser pulse has to be very short (o1 ms) and the 337
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emissions. Photodiodes and accelerometers are the most effective and are reported to have a low failure rate in comparison with the others (Tangermann and Uller, 2001). The aim of the present study was to evaluate these two sensor systems for the regulation of the laser ablation process in vitro on bone of the rabbit femur and the jaw of minipigs.
MATERIAL Laser and sensor system The Erbium:YAG-laser (‘‘Glissando’’ type, WaveLight, Tennenlohe/Erlangen, Germany) was used to deliver an energy range from 300 to 2000 mJ at pulse rates of 1, 10, 15 and 20 Hz at a pulse duration of 350 ms. For fast processing of the data controlling the laser osteotomy, a computer with integrated digital signal processor (DSP) was used. A silicium-photodiode and a piezo-electric accelerometer were chosen as sensors (Fig. 1) in order to measure the emitted optical and acoustic signals caused by the interaction of laser light with bone and other biological structures in the region. On the one hand, the sensor system has to be robust enough to withstand sterilization, whereas on the other hand it needs to be as small as possible for surgical use. The laser beam passed a beam splitter and an optical lens. The distance to the osteotomy site was defined via a stainless steel pin of defined length parallel to the laser beam. Reflected optical signals passing through the lens and beam splitter were registered. An optic fibre transmitted these signals to a photodiode where they were transformed into electric signals and processed in the digital signal processor (DSP). The fibre and the beam splitter were integrated in an applicator system. The other process emissions were caused by structure-borne sound and
detected by a piezo-electric accelerometer. This detector was mounted on bone with a holder fixed with two screws from a conventional miniplate osteosynthesis system. The induced electric signal was processed in the DSP. The signals were processed at the same time by an analogue–digital converter with a sampling rate of 330 kHz. The recording of these signals was triggered by the TTL-signal of each laser pulse; 128 data bits were sampled per pulse. Characteristic time-dependent values for the electrical signals released by the photodiode and the accelerometer (amplitude, standard deviation) were defined. Additionally, characteristic frequency values (Fourier transformation) were defined for the accelerometer. If the DSP detected a change in the signal character in only one of the two signals recorded, the laser-beam cutting process was interrupted by the closed-loop control system (Fig. 1). The integrated system evaluated not only the amplitudes of both sensor systems but also the detected frequency changes of emissions caused by each pulse were compared via a Fourier transformation. A change of frequency or wave character switched the laser off.
METHODS Surgical procedure After establishing a working, closed-loop control system for controlling the laser on several bone probes of rabbit femur and minipig jaw (these bone probes were easily available in the animal laboratory of the department of Oral & Cranio-Maxillofacial Surgery) a controlled in vitro study was performed on 14 dissected femurs of New Zealand white rabbits, and six lower jaw halves of Goettingen minipigs. The laser ablation or bone cutting was performed under water-spray cooling to improve the absorption of laser radiation by the bone and thus to prevent thermal interactions like carbonization in the region adjacent to the laser-cut. The accelerometer signal system was fixed onto the bone specimens, then the bone was cut by the Er:YAG laser. Rabbit femur was chosen because of the tubular structure with a strict border between bone and bone marrow. The mandibles of minipigs are of a more complicated anatomical structure including tooth roots and the differing structure between cortical and cancellous bone, and the neurovascular bundle has to be preserved. Five holes were cut in the rabbit femurs, whilst in the pig mandibles, 9–10 holes were made. Evaluation
Fig. 1 – Circuit of closed-loop control system 1=Er:YAG laser beam; 2=beam divider; 3=process emissions; 4=bone specimen; 5=optic fibre; 6=photodiode; 7=piezoelectric accelerometer; 8=digital signal processor.
The specimens were evaluated macroscopically and histologically. Bone samples were preserved in a 7% formalin solution. Following this, the specimens were dehydrated for 2 days each, in alcohol solutions with ascending concentrations of 50, 70, 90 and 100%.
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Finally, the bone specimens were embedded in lightcuring resin (Technovit 7200 VLC, Kulzer, Werheim, Germany) and hardened in a heating cupboard (Exact, Norderstedt, Germany) for 2 days. The specimens were sliced at approximately 300 mm thickness using the Exact-cutting system (ExactApparatebau, Norderstedt, Germany). The samples were then ground to a thickness of 30 mm and stained in toluidine blue O (Chroma, Muenster, Germany) according to Donath (Donath, 1988). The cross sections were cut parallel to the laser cut canals and have been evaluated with a ‘Quantimet 500’ by Leica (Germany). The depth of each hole was calculated as a percentage of the whole thickness of cortical bone (Fig. 2). The scanned cross sections were marked for the thicknesses of cortical bone layer and real cortical ablation depth.
RESULTS Applying the closed-loop controlled laser system resulted in an automatic interruption of the laser beam as soon as cancellous bone or bone marrow was reached. The reflected sound and the optical emissions changed dramatically as soon as the cortical bone was transected by the laser beam. The voltage signal of the piezo-electric accelerometer detected increased amplitudes of 2.12 V (SD=0.27 V); on the site of the photodiode 7.48 V (SD=0.46 V) was measured. This is demonstrated in Fig. 3. Fig. 3B illustrates the signals detected by the working control system. The changed sensor-signal stopped the subsequent laser pulse. The osteotomy was stopped by the accelerometer in rabbit/minipig bone in 55.7/ 60.3% respectively by photodiode system in 44.3/ 39.7%. Inspection of the bone specimens from the rabbit femur showed only a small perforation of the cortical bone layer bordering the bone marrow. The size of the perforation was not more than about a quarter of
the width of the area of laser beam ablation, but was wide enough to be detected by the sensor systems (Fig. 4A). The cortical bone layer was not exactly even. Perforation was then restricted to the area that was thin enough to be cut by the last emitted pulse before the laser was switched off. On greater magnification, a small perforation of the cortical bone could be seen (Fig. 4B). A thin cortical layer covered the surrounding area indicating that the laser beam was turned off right at the precise moment of cortical perforation. Histological examination (toluidine blue staining) confirmed this (Fig. 5). In the minipig jaw, no damage to the nerve bundle, nor the perineurium was found histologically. Histomorphometrical evaluation of the hole depth in the 14 dissected rabbit femurs and the six halves of pig mandible showed a mean ablation rate for cortical bone of 97.45% in pig; (median=99.5; SD=3.19; n=58), and 97.83% in rabbit; (median=100; SD=2.74; n=70; Fig. 2B, Table 1). There were no values detected over 100%. There was no difference between the models (pig mandible–rabbit femur). No damage to the alveolar nerve was seen on any cross section due to the fact that the laser was stopped at the 100% mark of cortical bone at the latest.
DISCUSSION Excimer laser systems had shown good properties in treating bony structures (Kochevar, 1989; Lustmann et al., 1991). Excimer lasers are less efficient in the bone ablation rate per pulse when compared with an Er:YAG laser. The Er:YAG laser has proved to be an optimal system for bone cutting (Lewandrowski et al., 1996; Peavy et al., 1999) as it displays the highest bone ablation rates per pulse for effective cutting with little or no carbonization on the adjacent bone margins, if adequate water cooling is applied (Lewandrowski et al., 1996; Kimura et al., 2001).
Fig. 2 – (A) Schematic drawing of evaluation for the depth of each laser hole. The depth was set relative to the cortical layer (=100%) illustrated as the grey bar covering the cancellous bone/bone marrow. The histological cross section above shows two holes in the cortical layer. (B) Mean cortical bone ablation rates (%) in minipig jaw (n=58 holes) and rabbit femur (n=70 holes): results are illustrated as box plots, the corresponding values are given in Table 1. The cortical ablation is very accurate for ablation of only the cortical bone layer.
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Fig. 3 – (A) Voltage signal produced by the piezo-electric accelerometer as a function of time and pulse number (Fourier transformation). (B) Voltage signal analogue to Fig. 2A demonstrating the working system of the closed-loop control system. Shut down occurred after the change of signal character. (C) Voltage signal of the photodiode as a function of time and pulse number (Fourier transformation).
Fig. 4 – (A) Rabbit femur. The diaphysis was cut four times. On the right hand three holes, a small part of the cortical bone was perforated – consequently the laser beam was turned off by the control systems. (B) Magnification of the right holes. On the left, a small cortical layer covers most of the bone marrow.
Peavy et al. (1999) also tested laser systems working within the infrared wavelength of 2.9, 3.0 and 5.9– 6.45 mm and found them to be ideal for bone ablation. Clean ablation surfaces free of carbonization debris were revealed. In the past, various laser
systems have been studied revealing problems related to the aspect of bone cutting. The Er:YAG laser is the only laser system offering good bone ablation at each pulse without carbonization. It shows only minimal thermal damage.
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Fig. 5 – (A) Histology of the minipig jaw. Laser cut stopped in front of bone marrow. The hole in the middle shows a small cortical layer where the laser was stopped. The neurovascular bundle is intact (toluidine blue, 1). (B) Histology of the laser hole at transition from cortical to cancellous bone of minipig jaw. Laser stopped at the transition from cortical to cancellous bone (toluidine blue, 25).
Table 1 – Cortical bone ablation of minipig’s jaw and rabbit femur given as percentage of the whole cortical bone layer
n Mean Median Standard deviation Maximum Minimum
Minipig jaw
Rabbit femur
58 97.45 99.50 3.19 100 88
70 97.83 100 2.74 100 85
It could be employed in numerous medical fields, e.g. in cranio-maxillofacial surgery, neurosurgery or orthopaedic surgery. A closed-loop control system is essential when complicated anatomical areas are irradiated by a laser beam to avoid tissue damage especially to neural structures or blood vessels in the direct vicinity or within the bone.
CONCLUSION Lewandrowski et al. (1996) showed that the bone healing process after laser ablation was comparable to mechanical bone cutting. By comparing laser and mechanical osteotomies no histological differences concerning the bone healing process could be detected in the rat mandible model. Precise control of the bone cutting is possible as the laser ablation offers osteotomy without physical contact. To make the laser osteotomy reliable and safe for the surrounding tissue structures – e.g. nerve bundles, salivary glands and ducts or vessels – a control system had to be developed to prevent the laser energy from ablating tissues other than bone. For the first time, a closed-loop control system has been developed, and it has been demonstrated that the processing of optical and acoustic emissions was a very sensitive method to differentiate between cortical and cancellous bone, or between bone and soft tissues like bone marrow. This could be documented macroscopically, via X-ray and especially histomorphometrically. Both sensor systems signals changed immediately when penetrating the cortical bone layer. (The consequence would be that for example a nerve bundle would be damaged in its canal.) However, the sensor signals interrupted the laser beam as soon as threshold values were reached. This control concept was established in the laboratory, applied in an in vitro model and now has to be refined for application in an in vivo model.
It was demonstrated that a highly selective ablation of bone is possible by means of an Er:YAG laser. In this in vitro model the border between cortical and cancellous bone was detected when penetrating the cortical layer by applying a closed-loop control system. Such system might effectively reduce the risk of damaging surrounding or contained soft tissues during laser osteotomies.
Acknowledgements The sensor controlled laser is an interdisciplinary project which is funded by the VW-Stiftung (AZ: I/73 770, 773-775).
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[email protected] Paper received 4 July 2002 Accepted 15 July 2003
